Depends on what kind of logbook you have. Some have a section with 'other flying' where you could record it. Alternatively you could order a glider pilots logbook from the BGA website. Will only cost you about £1.50. My one is over 15 years old now but i dont think the format has changed. Cost me about 50 pence when I first got it and I still use it to this day. If in any doubt then contact PLD at CAA and ask for their opinion on how best to log the gliding time.